The present invention relates to the field of heating, ventilating and air conditioning ("HVAC") and, more particularly, to sheet metal ducting used to channel air within a building structure thereby to effect the heating and cooling thereof.
Many factors must be balanced when selecting the type and sizing of duct for a particular building installation. One principal factor is the volume of air that must be delivered and the pressure at which such delivery shall be made. But other issues are of substantially equal importance including, for example, the physical space limitations within the available ceiling drops, walls, and riser columns. In short, the duct must comport with the structural realities of the building which realities include making accommodation for the other utilities (e.g. electric, water, sewage) that must share the available volume.
Finally, although potentially at the top of the `factors list`, is cost. HVAC ducting is no exception to the law of nature which provides that `that which works the best, costs the most` and that `which doesn't cost, doesn't work`. Nature notwithstanding, the technology of the present invention `bends` nature's law by combining the best in functionality within the confines of affordable fabrication.
The standard of the HVAC industry has been, and remains, the rectangular duct. The rectangular form has numerous advantages that frequently makes it the duct of choice. First, rectangular duct provides the greatest cross-sectional area (i.e. air carrying capacity) of any of the duct families for any given maximum width or height dimension. By reason of the limited size of most duct races, this is not a trivial consideration. Probably the single greatest feature of the rectangular duct family, however, is its simplicity, ease of manufacture and corresponding economy of its fittings. Virtually every conceivable transition and fitting can be fabricated in the rectangular format and done so inexpensively--often with only four pieces, which pieces may be computed and automatically cut from sheet metal stock (see, for example, U.S. Pat. No. 4,551,810 to Levine). Further, the edges of the several constituent pieces that define the fitting may be brake or machine-formed to create a Pittsburgh, snap-lock or similar seam whereby the fitting may be expeditiously assembled without resort to welding or similar time-consuming attachment measures.
For all of its advantages, rectangular duct has a few significant short-comings including, for instance, the short lengths of duct sections available and the relative complexity of manufacture of such straight sections. To appreciate these limitations it is necessary to remember that straight rectangular duct sections are conventionally fabricated from coil stock typically five feet in width. While straight sections of virtually any length could, in theory, be made simply by rolling out sufficient stock, only certain size ducts may be thus fabricated within the five foot width constraint without substantial sheet metal waste. Thus, straight sections are fabricated, not by uncoiling long lengths of sheet metal, but by forming the requisite component pieces by a series of transverse cuts. In this manner there is little waste, but the tradeoff is duct the maximum length of which is the width of the coil stock employed.
Circular and the so-called `flat-oval` duct families solve the above straight-section length problem while, not surprisingly, introducing yet other problems. Circular duct is literally wound as a solid helix from a relatively narrow coil of about six inches in width. The diameter of the finished circular duct is that of the solid helix and may be any reasonable diameter. The advantage of this duct configuration is the absence of length restrictions such as found with rectangular duct, at least aside from the obvious practicalities of handling excessively long duct sections. Any length duct may be fabricated by continuing the helical winding process until the desired length is reached.
Circular duct, however, is the least efficient in terms of conveying air volume, at least when confronted with a finite space limitation--the usual situation. A circular duct must be as high as it is wide and therefore greater air carrying capacity cannot be achieved simply by making the duct wider. And it will be further appreciated that even to the extent of the available height dimension, circular duct `fills` this full height only in the center thereof, leaving unused dead spaces in what would be the corners of a rectangular (square) duct of the same lineal cross-sectional dimension. Rectangular duct, by contrast, fills the full available height space across its entire width.
Flat-oval duct was developed to solve this inefficiency of circular duct while maintaining its simplicity of construction and length versatility. Indeed, one popular method for the manufacture of flat-oval duct is to first `wind` a circular duct section, thereafter, to `ovalize` it by placing it on a pair of mandrels which are, in turn, forced outwardly, transverse to the longitudinal duct axis, to form an oval or flattened circular duct. Flat-oval duct comes closer to approximating the air carrying capacity of rectangular duct while not, as noted, sacrificing the advantages obtained from circular duct.
But again, all is not perfect--with one attribute there appears always to be an offsetting detriment. And flat-oval is no exception. Fittings for flat-oval duct are considerably more difficult to manufacture than corresponding rectangular fittings. A typical flat-oval fitting is comprised of plural "gore" sections, each of which must be carefully formed into an oval contour from an irregularly-shaped cut-out. The continuously curved edges that define these gore sections renders the machine formation of seams difficult and expensive. Thus, typically, the resulting gore sections are welded.
As a consequence of this tortuous fabrication scenario, flat-oval has not attained the popularity that might otherwise have been anticipated and, to the extent used, it has been found that most contractors do not fabricate fittings `in-house` as they do with rectangular duct but, instead, must obtain such fittings as purchased parts from outside vendors. This is not only more expensive, but the contractor looses control over fitting quality and delivery and cannot make last minute adjustments as may, for example, be required in response to unforeseen job site complications.
As noted above, the present invention seeks to revise the natural law that `that which works best, costs most` by combining the best of the rectangular and flat-oval duct families in a manner that the beneficial attributes of each family are realized without the high cost ordinarily associated with such superlative performance.
More specifically, the present invention relates to a family of modified rectangular fittings, referred to herein as "otangular" fittings, which facilitates the interconnection of flat-oval duct with an ease and cost comparable to conventional rectangular fittings. It will be appreciated that this otangular technology fuses the essentially limitless length capability of flat-oval straight-sections with the ease of manufacture and corresponding cost efficaciousness of rectangular fittings.
Otangular fittings have other advantages as well. For example, certain transitions and fittings virtually unheard of in flat-oval duct systems (e.g. a drop cheek elbow) is as easily fabricated using otangular technology as it would be employing a conventional rectangular fitting. Yet another advantage of otangular fittings is the ability to fabricate mixed fittings which mate rectangular duct, on one end, to flat-oval, on the other end.
The beauty of otangular technology is its ease of implementation wherein modified rectangular fittings are combined with--what will become `off-the-shelf` "oval-corners"--to complete the fitting. As noted, most rectangular fittings are fabricated using automated machinery in which the shapes of the various flat sheet metal pieces required for the subject fitting are automatically computed, oriented or "nested" on sheet or coil stock, and cut therefrom. Automated machinery is again employed, this time to form seams along appropriate edges as required to assemble the completed fitting. This consummate ease of automated, machine-based fabrication represents an important advantage of rectangular duct technology--an advantage that can now be extended to flat-oval systems through the present otangular family of fittings.
And it cannot be overlooked that the cost of such automated machinery has fallen to where most HVAC installation shops can now afford to own and operate the equipment themselves. They can, in short, control their own fabrication schedules, make changes are required, and reap the profits from their manufacture or pass the savings to the customer as, frankly, may be required to remain competitive.
Otangular fittings are similarly suited for manufacture in small and large HVAC shops alike utilizing existing automated HVAC layout and seam-forming equipment. The initial fabrication process will be substantially identical to that now employed except that the computerized software shape determination, nesting, and cutting routines will be slightly modified, as outlined in more detail herein, to reflect the otangular duct configuration, in particular, the interface with the above-mentioned oval-corner pieces, a pair of which is required at each fitting interface to effect an interface to conventional flat-oval duct.
Fabrication routines for standard rectangular fittings generally require modification only to the extent of, first, forming the ends of the two parallel sides that define the `major axis` of the fitting inlet/outlet to thereby create a tapered trapazoidal end region having, on the respective outside edges thereof, a predetermined taper angle and, second, dimensioning the remaining two sides (that define the `minor axis`) such that the ends thereof coincide with the inside end of the above-noted taper angle whereby two oval-corner receiving regions are defined on each end of the otangular fitting.
It is important to note that this basic modified rectangular fitting includes nothing not otherwise required to fabricate a conventional fitting of rectangular form, in particular, there are no special angles, bends or additional members--often just four pieces, as in a conventional fitting assembled using conventional seams and construction techniques. As thus assembled--and without inclusion of the oval-corners discussed hereinafter--the completed modified duct looks very similar to that of its unmodified rectangular counterpart except that the inlets and outlets thereof, as described above, have opposed recesses adapted to receive the requisite pairs of oval-corner components.
As outlined hereinafter, the fabrication of otangular fittings employing attachable oval-corner members offers several important advantages and is therefore the preferred approach. For instance, while it may initially appear that the number of duct sizes is virtually limitless, in reality there are only a relatively few "families" of flat-oval duct. As used herein, a duct "family" refers to all duct sizes having a given minor axis dimension, thus, for example, the 12" family would include the following flat-oval ducts: 12".times.18", 12".times.24", 12".times.36", 12".times.48" etc. It is significant to the efficacy of the present invention that all members of any given duct family may be fabricated using the same oval-corner member. And for this reason it will be appreciated that implementation of the full range of otangular fittings--and, implicitly, the full range of flat-oval duct sizes--requires only a few standard oval corner designs.
Herein lies one of the principal advantages of the otangular system of flat-oval fittings. The relatively few standard sizes of oval-corners can be tooled and stamped out in large, production quantities and sold, very inexpensively, as `purchased items` to individual contractors. The comparatively few sizes required coupled with the economies of high volume, tooled production suggests that most contractors will choose to purchase and stock their oval-corner requirements as opposed to the in-house manufacture thereof.
In this manner the labor-intensive steps in fabricating an otangular fitting--i.e. the multiple sheet metal bends necessary to approximate the circular contour of the flat-oval duct--are completely eliminated and replaced, instead, by the simplist of installations of the purchased and inexpensive oval-corner members.